Salt Sensitivity and Hypertension in African Americans: Implications for Cardiovascular Disease

Rosalind M. Peters, MSN, RN, John M. Flack, MD, MPH

Disclosures

Prog Cardiovasc Nurs. 2000;15(4) 

In This Article

Clinical Implications

There are four treatment priorities to help reduce the occurrence of complications associated with salt sensitivity hypertension (Table III). These include control of systolic blood pressure (SBP), dietary interventions, weight reduction, and adjustments in drug therapy.

The predominant impact of dietary sodium intake is on SBP, which is more closely correlated with cardiovascular risk than diastolic pressure (DBP).[2,3,34] In fact, isolated systolic hypertension (SBP ≥160 mm Hg and DBP <90 mm Hg), which is especially prevalent in African American women after age 55, is associated with a two- to five-fold increase in cardiovascular morbidity and mortality, even though DBP is normal.[3] The importance of SBP was emphasized in two recent reports from the Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure (JNC).[2,35] These reports present a classification schema of blood pressure based on the concept of risk. A category of "high-normal" was added to emphasize the fact that persons with SBP or DBP not in the "hypertensive" range still have an increased risk of target organ damage, which can contribute, over the long term, to fatal and nonfatal cardiovascular events. Recognizing that target organ damage can occur with pressures below the hypertensive range is an especially important consideration for nurses for two reasons.

First, African Americans, especially women, are at high risk of developing LVH. The relationship of LVH to blood pressure is continuous, occurring across a broad range of blood pressure but beginning well within the normal range. Obesity, which is disproportionately prevalent in African American women, has been found to be the most important factor in the development of LVH, as well as in the development of salt sensitivity and hypertension.[3]

Second, in the U.S., SBP rises progressively in both men and women throughout adulthood. The age-related rise is more pronounced for systolic than for diastolic pressure, with DBP declining after the sixth decade of life.[1] Older people have been shown to manifest more salt sensitivity than younger persons. However, the importance of salt intake in these data is revealed in epidemiologic studies that show virtually no age-related increase in blood pressure in populations who consume <100 mmol of salt per day.

According to the JNC VI,[2] lifestyle modifications are especially important treatment modalities for hypertensives, and particularly for African Americans. This is due not only to their high prevalence of salt sensitivity, but also to the high prevalence of obesity, physical inactivity, and noninsulin-dependent diabetes within this population.

A recent meta-analysis of 56 dietary sodium reduction trials revealed that sodium restriction resulted in reduced SBP.[36] The greatest blood pressure response to dietary sodium interventions was noted in older, hypertensive subjects. The authors questioned the effectiveness of dietary sodium restriction in normotensive subjects, as their analysis found only small and nonsignificant changes in blood pressure of normotensive subjects.[36] However, it should be noted that the median length of intervention was only 2 weeks in the normotensive studies, which may have been an insufficient duration of time for salt-induced blood pressure changes to be fully manifest. Another meta-analysis revealed that moderate sodium reduction lowers SBP and DBP in both hypertensive and normotensive subjects.[37] Other studies suggest that decreased dietary sodium intake not only reduces blood pressure, but also may be associated with a reduced need for antihypertensive drug therapy, decreased diuretic-induced potassium loss, and regression of LVH.[2,37]

It has been estimated that dietary sodium restriction reduces blood pressure in 30%-60% of hypertensive subjects and in 25%-40% of normotensive subjects.[29] Therefore, moderation of dietary sodium intake remains an important treatment option in controlling hypertension, and may also be useful in its prevention.[37] However, the changing trends in salt ingestion in the U.S. make this difficult to accomplish. Health education programs in the late 70s and early 80s were very effective in getting American consumers to reduce their use of table salt. This decline was so significant that salt shaker usage is estimated to contribute as little as 2%-10% of total sodium intake today.[38] However, due to lifestyle changes, such as an increased number of women in the workforce and smaller families, approximately 40% of the American food dollar is spent on food away from home. Eating fast foods or high sodium convenience foods exposes persons to a high level of salt that is difficult to control at the individual level.[38] However, clinical trials have demonstrated the feasibility and acceptability of reducing sodium intake to <100 mmol/day.[35,37,38] This level of reduction in sodium intake is associated with a 2.2-10 mm Hg decrease in SBP.[35] Therefore, assisting hypertensives and persons with high-normal blood pressure in planning meals that take into account "hidden" sodium in convenience and restaurant foods is an important nursing intervention. Another important nursing intervention is to assist clients in understanding the relationship between sodium and salt (or sodium chloride). Table IV is a conversion chart that shows that 1 tsp of table salt is equivalent to 2 gm Na and 6 gm NaCl.

Interventions that include increasing dietary potassium intake may also be beneficial to salt-sensitive patients. Increased potassium intake in the setting of a high sodium diet confers some protection against a rise in blood pressure, as potassium augments sodium excretion. The JNC VI[2] recommends an intake of 50-90 mmol of potassium per day from dietary sources such as fresh fruits and vegetables. The role of potassium becomes especially important in hypertensive patients who are treated with potassium-losing diuretics, as those drugs may compound the potassium-salt sensitivity relationship. In addition, dietary potassium intake appears to lower blood pressure in individuals consuming high levels of dietary sodium. This most likely occurs via potassium's augmentation of urinary sodium excretion.

Low levels of calcium intake have been found to amplify the effects of a high sodium diet on blood pressure. In addition, increased calcium has been observed to reduce blood pressure in some patients. However, the JNC VI[2] does not believe there is sufficient evidence to indicate the use of supplemental calcium in the treatment of hypertension. Patients should, however, ensure that they are taking the current recommended daily allowance of 800-1200 mg of calcium.[35] Since lactose intolerance is more prevalent in African Americans, they may avoid dairy products and be at risk for calcium intake that does not meet the minimum recommended standards.

A combined intervention of dietary sodium reduction and weight loss may provide the greatest lifestyle modification benefit to African Americans. Evidence indicates that salt sensitivity can be ameliorated with weight loss.[17] Given the high prevalence of both obesity and hypertension in African Americans, especially women, combined programs would seem to provide the best results. However, care must be taken not to create an "overload" of information with too many complex messages, which could negatively influence the results.[38] In addition, all dietary interventions should be culturally sensitive and presented in a manner that is relevant to the needs and/or lifestyles of the clients.

African Americans appear to respond differently in a quantitative, rather than a qualitative, sense to selected drug therapies for hypertension. This occurs in part because of the higher prevalence of salt sensitivity, which contributes to a need for higher drug doses to achieve a given magnitude of blood pressure lowering. Diuretic and calcium antagonist monotherapies have been found to be more effective, on average, in lowering blood pressure in salt-sensitive subjects on a high sodium diet. Monotherapy with ß blockers or ACE inhibitors has been less effective than diuretics or calcium antagonists in African Americans when given at low doses in the setting of unrestricted sodium intake. Nevertheless, up-titration of the drug dose, a decrease in dietary sodium intake, or the addition of a diuretic improves the blood pressure response to these drugs.[2] A high level of dietary sodium intake augments renal protein excretion and attenuates the antiproteinuric effect of even profoundly antiproteinuric drugs, such as ACE inhibitors.

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